The Biomarkers in autism of aripiprazole and risperidone treatment (BAART) project will provide evidence-based guidance in the selection and monitoring of drug treatment of autism. BAART involves three academic centers across South Carolina, with expertise in phenotyping patients with autistic spectrum disorders, assessing patient response in clinical trials, and expertise in pharmacogenomic research. Although the FDA has approved use of the antipsychotic drug risperidone for irritability associated with autistic disorder, a moderate response rate in pivotal clinical trials and concerns over tolerability and weight gain can force clinicians to select alternative drug treatments, for which evidence-based support is sparse. BAART will assess predictors of efficacy, tolerability, and safety in 200 children five to seventeen years old with autistic disorder (AD) during a double-blind, randomized ten week treatment period with either risperidone or aripiprazole following a two-week single-blind placebo period. Responders who complete the study may continue with medication treatment for three months. Factors considered will include 1) psychiatric history;2) symptom response;3) psychosocial support;4) measures of tolerability;5) serum prolactin and brain-derived neurotrophic factor concentration;and 6) a variety of single nucleotide polymorphisms related to target genes for drug disposition and transport, response, and tolerability. Data analysis will allow assessment of gender, race, and multiple biomarkers for their predictive value on the clinical course and outcome to treatment with two widely used pharmacologic interventions for AD. The BAART project will result in evidence-based guidelines for selection and monitoring of drug treatment of children and adolescents with AD. PROJECT NARRATIVE: Autism occurs in approximately 1 in 150 children. The investigators propose a multi-center clinical trial to provide evidence-based guidance in the selection and monitoring of durg treatment of autism.